|Publication number||US6100544 A|
|Application number||US 09/081,760|
|Publication date||Aug 8, 2000|
|Filing date||May 20, 1998|
|Priority date||May 20, 1998|
|Publication number||081760, 09081760, US 6100544 A, US 6100544A, US-A-6100544, US6100544 A, US6100544A|
|Inventors||Kun-Chuan Lin, Lung-Chien Chen|
|Original Assignee||Visual Photonics Epitaxy Co., Ltd.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (9), Referenced by (11), Classifications (10), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates to a light-emitting diode (LED) having a double hetero structure which contains a second cladding layer with a graded composition, and to a light-emitting diode containing an upper semiconducting layer with a graded composition for increasing the brightness and decreasing the forward bias voltage (Vf).
Light emitting diodes (LEDs) having a semiconductor light generation region situated on a light absorbing substrate are widely used as light sources and are beginning to replace incandescent lamps. In order to meet the demands for light output, it is important that the overall light output efficiency of the LEDs be maximized. One improvement made in the prior art, disclosed in U.S. Pat. No. 5,008,718 is to increase the LED electrical efficiency by including a thin electrically conductive transparent window layer between the light generation region and the top metal contact so that current crowding is minimized. An unfortunate shortcoming of the thin window layer is that a significant portion of the total light generated from the light generation region is internally reflected within the window layer and is absorbed by the substrate instead of being emitted out of the LED. U.S. Pat. No. 5,233,204 disclosed a light-emitting diode with a thick transparent layer. However, the disclosed LED still has a Vf value greater than 1.9 volts, which means that the working voltage of the LED is too high to be used in current portable electrical equipment, such as cellular telephone, notebook computer, portable stereo, etc. It consumes a large amount of electrical energy.
It is an object of the present invention to provide a light-emitting diode having a double hetero structure which contains a second cladding layer with a graded composition of (Alx Ga1-x)y In1-y P in which 0.05≦x≦0.9 and 0.1≦y≦0.95. A lower cladding layer (Alz Ga1-z)0.5 In0.5 P (0.3≦z≦0.9) is first grown on top of a GaAs substrate. An active layer (Alw Ga1-w)0.5 In0.5 P (0≦w≦0.5) is grown above the lower cladding layer. The second cladding layer is then formed on top of the active layer. By means of the second cladding layer of (Alx Ga1-x)y In1-y P, the emitted of light from the surface of the light-emitting diode increases and the crowding effect is improved. The Vf value of the LED device of the present invention is equal to or less than that of conventional LEDs.
It is another object of the present invention to provide a light-emitting diode which contains an upper semiconducting layer with a graded composition of (Alx Ga1-x)y In1-y P, wherein 0.05≦x≦0.9, 0.1≦y≦0.95. The light-emitting diode contains a substrate, a lower semiconducting layer grown on top of the substrate, the upper semiconducting layer having a graded composition of (Alx Ga1-x)y In1-y P, and two ohmic contacts. By means of the upper semiconducting layer with a graded composition of (Alx Ga1-x)y In1-y P, the brightness of the LED increases and the Vf value of the present invention is equal to or less than that of the conventional LEDs.
FIGS. 1A and 1B are cross-sectional views of first and second conventional light-emitting diodes.
FIGS. 2A and 2B are cross-sectional views of third and fourth conventional light-emitting diodes.
FIG. 3 is a cross-sectional view of a first embodiment of light-emitting diodes of the present invention.
FIG. 4 is an energy band diagram of the first embodiment of FIG. 3.
FIG. 5 is a cross-sectional view of a second embodiment of light-emitting diodes of the present invention.
U.S. Pat. Nos. 5,008,718 and 5,233,204 disclose light-emitting diodes having a transparent window layer which improves the crowding effect of conventional LEDs and increases the amount of the emitted light from the surface of the LEDs. Thus, the brightness of the emitted from light the LEDs significantly increases.
FIGS. 1A and 1B are cross-sectional views of conventional LEDs. FIG. 1A is a homo structure of an LED which comprises a substrate 110, a first ohmic contact 114 to the substrate 110, a first type conductivity material 111, such as n-AlGaInP, formed on top of the substrate 110, a second type conductivity material 112, such as p-AlGaInP, formed on the first type conductivity material 111, and a second ohmic contact 113 to the second type conductivity material 112. FIG. 1B shows a double hetero structure of a conventional LED which is similar to that of the LED of FIG. 1A except that an active layer 125 is formed between a second type semiconducting layer 122, such as p-AlGaInP, and a first type semiconducting layer 121, such as n-AlGaInP. The energy gap of the active layer 125 is smaller than those of the first and second type semiconducting layers 121, 122 so as to provide a carrier confinement effect to confine electrons and holes in the active layer 125. Thus, the electrons and holes recombine in the active layer 125 to emit light. The first and second semiconducting layers 121,122 are named as lower cladding and upper cladding layers. The external quantum efficiency of the above LEDs of FIGS. 1A and 1B is not high because of some factors, such as, current crowding effect, incident light critical angle, re-absorbing inside the LEDs. Therefore, U.S. Pat. Nos. 5,008,718 and 5,233,204 disclose LED structures having a transparent window layer in order to increase the external quantum efficiency of the LEDs as shown in FIGS. 2A and 2B. In FIGS. 2A and 2B, transparent window layers 216 and 226 are grown respectively on the LEDs of FIGS. 1A and 1B. The materials of the transparent window layers 216 and 226 are Gap, GaAsP, or AlGaAs which have energy gaps greater than that of AlGaInP light-generating layer. Although, by means of the transparent window layer, the brightness of the LEDs increases, and the forward bias voltage Vf increases because a bandgap discontinuity problem (ΔEc and ΔEv) arises due to a hetero junction which is formed between the transparent window layer and the upper cladding layer or the second type semiconducting layer of the LED. Thus, the power consumption of the LEDs increases. The forward bias voltage Vf is defined as the voltage value measured when a forward current of 20 mA is applied to the LEDs. In order to solve the bandgap discontinuity problem of large Vf value, the present invention provides the LED having an upper cladding layer of a graded composition.
In FIG. 2A, the numeral 210 is a substrate. Numeral 213 is an ohmic contact. Numeral 214 is an ohmic contact. Numeral 212 is a second type conductivity material.
In FIG. 1B, the numerals 123 and 124 are ohmic contacts. Numeral 120 is a substrate.
In FIG. 2B, the numerals 223 and 224 are ohmic contacts. Numeral 222 is a p-type confining layer. Numeral 225 is an active layer. Numeral 221 is an n-type confining layer. Numeral 220 is a substrate.
In the present invention, all the layers, such as, a first cladding layer 311, a second cladding layer 317, an active layer 315, an upper semiconducting layer 517, a lower semiconducting layer 511, etc. are grown using metal-organic chemical vapor deposition (MOCVD). The temperature of MOCVD is from 500 to 750° C. and the pressure is from 100 to 300 mbar.
As shown in FIG. 3, according to the first embodiment of the present invention, the substrate 310 is an n-type GaAs. An n-type AlGaInP first cladding layer 311 is formed on the substrate 310. A p-type AlGaInP active layer 315 is grown on the first cladding layer 311. A p-type AlGaInP second cladding layer 317 having a graded composition is then formed on top of the active layer 317 in order to form a double hetero structure. In the first embodiment of the present invention, the n-type dopant is Te and the p-type dopant is Mg. The schematic view of the energy band of the first embodiment of LED is shown in FIG. 4 in which the dimension is not drawn to the scale of the layer thickness and energy gap of the real LED. Numeral 321 is the conduction band and numeral 322 is the valence band as shown in FIG. 4. First and second ohmic contacts 314 and 313 are contact metal layers which are formed on the lower and upper surfaces of the LED.
In the first embodiment of the present invention, the thickness of the GaAs substrate 310 is between 250 μm and 450 μm, preferably 350 μmą20 μm. The typical thickness of the first cladding layer 311 is between 0.1 μm and 1.5 μm, preferably 0.8 μm. The thickness of the active layer 315 is between 2.8 μm and 0.9 μm, preferably 1.6 μm. The thickness of the second cladding layer 317 is between 2 μm and 15 μm.
In view of the composition of various layers of the first embodiment of the present invention, the first cladding layer 311 is (Al0.7 Ga0.3)0.5 In0.5 P and the active layer 315 is (Al0.3 Ga0.7)0.5 In0.5 P. The composition of the second cladding layer 317 is gradually changed from (Al0.7 Ga0.3)0.5 In0.5 P at the junction between the active layer 315 and the second cladding layer 317 to (Al0.1 Ga0.9)0.9 In0.1 P at the junction between the second cladding layer 317 and the second ohmic contact 313. A lattice match occurs between (Al0.7 Ga0.3)0.5 In0.5 P or (Al0.3 Ga0.7)0.5 In0.5 P and the GaAs substrate 310. The lattice mismatch between the (Al0.1 Ga0.9)0.9 In0.1 P and the GaAs substrate 310 is about 2.78%. Generally, the greater the thickness of the second cladding layer 317 is, the better the current spreading effect of the LED is. Thus, the amount of light emitted from the edge of the LED increases. However, if the other factors, such as, growth time and cost of the second cladding layer 317, are considered, the thickness of the second cladding layer 317 should not be very thick. Preferably the thickness is between 2 μm and 30 μm.
The active layer 315 of the first embodiment of the present invention is a well-known layer which may be a single layer quantum well (SQW) or multiple layer quantum wells (MQW). The active layer 315 may be n-type or p-type.
The AlGaInP layer having a graded composition may be applied to the LEDs with a conventional homo p-n structure or a single hetero structure. The manufacture process for the LEDs having a graded composition in the upper cladding layer is simple and cheap. If the upper layer representing the second conductivity type semiconducting layer is replaced by the AlGaInP layer having a graded composition, the brightness of the LED is improved and the Vf value is retained or decreased.
The second embodiment of the present invention is shown in FIG. 5. The LED of the second embodiment contains a GaAs substrate 510 of a first conductivity type, a lower semiconducting AlGaInP layer 511 of a first conductivity type grown on the substrate 510, and an upper semiconducting AlGaInP layer 517 of a second conductivity type AlGaInP grown on the lower semiconducting layer 511. A second ohmic contact 513 is deposited on top of the upper semiconducting layer 517. A first ohmic contact 514 is formed on the substrate 510. The composition of the upper semiconducting layer 517 of the second conductivity type is gradually changed from (Al0.3 Ga0.7)0.5 In0.5 P at the junction between the lower semiconducting layer 511 and the upper semiconducting layer 517 to (Al0.1 Ga0.9)0.9 In0.1 P at the junction between the upper semiconducting layer 517 and the second ohmic contact 513. The composition of the lower semiconducting layer 511 of the first conductivity type is (Al0.3 Ga0.7)0.5 In0.5 P.
In the present invention, the MOCVD technique is used. However, the layers may be formed by other techniques, such as, MBE, VPE, and LPE. Different materials, thickness and deposition conditions can also be used in the present invention.
While the invention has been particularly shown and described with reference to these preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. Although only the preferred embodiments of this invention were shown and described in the above description, it is requested that any modification or combination that comes within the spirit of this invention be protected.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US5003548 *||Sep 21, 1988||Mar 26, 1991||Cornell Research Foundation, Inc.||High power (1,4 W)AlGaInP graded-index separate confinement heterostructure visible (λ-658 nm) laser|
|US5008718 *||Dec 18, 1989||Apr 16, 1991||Fletcher Robert M||Light-emitting diode with an electrically conductive window|
|US5218613 *||May 1, 1992||Jun 8, 1993||Mcdonnell Douglas Corporation||Visible diode laser|
|US5233204 *||Jan 10, 1992||Aug 3, 1993||Hewlett-Packard Company||Light-emitting diode with a thick transparent layer|
|US5458085 *||Oct 19, 1994||Oct 17, 1995||Fujitsu Limited||Magnesium-doping in III-V compound semiconductor|
|US5714014 *||Jun 13, 1995||Feb 3, 1998||Showa Denko K.K.||Semiconductor heterojunction material|
|US5744829 *||Dec 26, 1996||Apr 28, 1998||Showa Denko K. K.||A1GaInP light emitting diode|
|JP40608538B *||Title not available|
|JP41101726B *||Title not available|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US6245588 *||May 11, 1999||Jun 12, 2001||Rohm Co., Ltd||Semiconductor light-emitting device and method of manufacturing the same|
|US6324200 *||Apr 20, 1999||Nov 27, 2001||Matsushita Electric Industrial Co., Ltd.||Semiconductor laser device|
|US6518159 *||Oct 19, 2000||Feb 11, 2003||Sharp Kabushiki Kaisha||Semiconductor laser device and a method for fabricating the same|
|US7863631||Apr 30, 2009||Jan 4, 2011||Koninklijke Philips Electronics N.V.||A1InGaP LED having reduced temperature dependence|
|US8507929 *||Jun 16, 2008||Aug 13, 2013||Koninklijke Philips Electronics N.V.||Semiconductor light emitting device including graded region|
|US8692286 *||Dec 14, 2007||Apr 8, 2014||Philips Lumileds Lighing Company LLC||Light emitting device with bonded interface|
|US20090152584 *||Dec 14, 2007||Jun 18, 2009||Philips Lumileds Lighting Company, Llc||Light emitting device with bonded interface|
|US20090230381 *||Apr 30, 2009||Sep 17, 2009||Koninklijke Philips Electronics N.V.||AlInGaP LED HAVING REDUCED TEMPERATURE DEPENDENCE|
|US20090309111 *||Jun 16, 2008||Dec 17, 2009||Koninklijke Philips Electronics N.V.||Semiconductor light emitting device including graded region|
|CN101897047B *||Dec 15, 2008||May 13, 2015||皇家飞利浦电子股份有限公司||Light emitting device with bonded interface|
|WO2006106467A1 *||Apr 3, 2006||Oct 12, 2006||Koninklijke Philips Electronics N.V.||Allngap led having reduced temperature dependence|
|U.S. Classification||257/94, 257/97, 438/47, 372/45.01, 257/103, 438/46, 257/96|
|May 20, 1998||AS||Assignment|
Owner name: VISUAL PHOTONICS EPITAXY CO., LTD., TAIWAN
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:LIN, KUN-CHUAN;CHEN, LUNG-CHIEN;REEL/FRAME:009194/0805
Effective date: 19980506
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Year of fee payment: 4
|Dec 19, 2007||FPAY||Fee payment|
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|Feb 14, 2009||AS||Assignment|
Owner name: HUGA OPTOTECH INC., TAIWAN
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:VISUAL PHOTONICS EPITAXY CO., LTD.;REEL/FRAME:022248/0979
Effective date: 20090203
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|Oct 13, 2016||AS||Assignment|
Owner name: EPISTAR CORPORATION, TAIWAN
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